Book/Report

FZJ-2018-04217

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Zur Wechselwirkung von NO3-Radikalen mit wässrigen Lösungen: Bestimmung des Henry- und des Massenakkomodationskoeffizienten

Thomas, K. (Corresponding author)FZJ* ; Volz-Thomas, A.FZJ* ; Kley, D.FZJ*

1993
Forschungszentrum Jülich GmbH Zentralbibliothek Verlag Jülich

Please use a persistent id in citations: http://hdl.handle.net/2128/19345

Report No.: Juel-2755

Abstract: The Henry coefficient of NO$_{3}$ radicals, K$_{h}$(NO$_{3}$), was studied in coiled denuder tubes of 2 mm diameter and variable length, e. g. 4 - 350 cm. The radicals were produced in a flow system by oxidation of NO with excess O$_{3}$ at a temperature of 393 K. A small flow of pure H$_{2}$O was metered to the gas flow and showed to form a thin film on the denuder wall. Behind the denuder, gas and liquid were separated and the concentration of the dissolved NO$_{3}$ was determined indirectly, after conversion to nitrate by reaction with Cl$^{-}$-ions, which were added to the effluent of the denuder. The partial pressure of NO$_{3}$ in the gas phase behind the denuder was determined by quantitative conversion to nitrate in another denuder. Determination of the Henry coefficient is complicated by reactions of the dissolved NO$_{3}$ radicals with OH$^{-}$. For this reason, the apparent Henry coefficient was found to increase as a function of the residence time of the solution in the denuder. In order to derive K$_{h}$(NO$_{3}$) from the measured NO$^{-}_{3}$-concentration and the partial pressure of NO$_{3}$, the aqueous-phase chemistry was simulated with a timedependent model. In these simulations K$_{h}$(NO$_{3}$) was varied between 0.5 and 100 mol $\cdot$ 1$^{-1}$ $\cdot$ atm$^{-1}$. The best agreement between the model and the experimental results was for K$_{h}$(NO$_{3}$) = 2 mol $\cdot$ 1$^{-1}$ $\cdot$ atm$^{-1}$ with an estimated uncertainty of a factor of 2. The same experimental set-up was used to determine the mass accommodation coefficient of NO$_{3}$ to an aqueous surface. There, a high C$^{-}$-concentration (0.1 M) was used in the stripping solution to efficiently convert the dissolved NO$_{3}$ to NO$^{-}_{3}$ and thus prevent phase-equilibrium to be reached. The mass accommodation coefficient was derived from the transmittance of the denuder as a function of length. The method requires the knowledge of the diffusion coefficient, which was determined in separate experiments conducted under the same conditions with HNO$_{3}$. The mass accommodation coefficient of NO$_{3}$ was found to be $\geq$ 2.5 $\cdot$ 10$^{-3}$. From the measured Henry coefficient and available thermodynamic information, the redox potential of dissolved NO$_{3}$/NO$^{-}_{3}$ was calculated to be in the range of 2.3 to 2.4 V. The data also allowed to estimate the rate coefficient for the reaction of dissolved OH radicals with NO$^{-}_{3}$. It was determined to be less than 10 l $\cdot$ mol$^{-1}$ $\cdot$ s$^{-1}$,more than four orders of magnitude smaller than the upper limit derived from pulseradiolysis studies. The relatively low solubility implies that heterogeneous losses of NO$_{3}$ radicals are of importance in the atmosphere only under conditions where sufficiently high concentrations of reactants, such as Cl$^{-}$, HSO$^{-}_{3}$, OH$^{-}$ or organic molecules are present in the liquid phase, to remove the dissolved NO$_{3}$. Theoretical considerations andtime-dependent box-model calculations show that even under such favourable conditions, heterogeneous losses of NO$_{3}$ are important for the budget of NO$_{x}$ only, if NO$_{x}$-concentrations are below 0.5 ppb. At higher NO$_{x}$-concentrations, the nighttime losses proceed predominantly via N$_{2}$O$_{5}$. In the maritime boundary layer, nighttime losses of NO$_{3}$ radicals can compete with NO$_{x}$ losses caused by OH radicals during day-time.

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Contributing Institute(s):

1. Publikationen vor 2000 (PRE-2000)

Research Program(s):

1. 899 - ohne Topic (POF3-899) (POF3-899)

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Database coverage:

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